The IUPAC standard defines colloidal particles, in general terms, as being particles with a size comprised between 1 nanometer and several micrometers. We refer to this definition whenever it uses the term “colloidal particles”.
Natural or synthetic fibers used as textiles or compounded extenders are often coated with additives. This coating has the aim of modifying the surface properties of the fiber or granting it a specific functionality. In certain cases, the term “bonding” can be used. For example, so-called “textile” bonding applied to filaments output from a spinneret consists of depositing a bonding agent ensuring the cohesion of the filaments with each other, reducing abrasion and facilitating subsequent manipulations (weaving) and preventing the build-up of electrostatic charges. There are other cases wherein a fiber must be covered with a specific compound. For example, it is possible to dye a fiber simply by coating it with dyeing agents. An initially insulating textile fiber can be made conductive by coating it with conductive polymers. It is possible to perfume a garment of clothing by coating its fibers with capsules containing a perfume.
Conventional fiber coating results in uniform, symmetric coating of its surface.
However, it is preferable in certain cases to add an additive to a fiber in a different manner, for example, on the surface, but in an asymmetric manner. These different conditions are proposed to improve the properties of the fibers, allowing them to be used for new functions.
Carbon nanotubes have a structure and electronic and mechanical properties which make them very promising materials for many applications: composites, electromechanical actuators, cables, resisting wire, chemical detectors, hydrogen storage, electron-emitting displays, energy converters, electronic components, electrodes, batteries, catalysis media, etc.
Carbon nanotubes can be made into ribbons or fibers by a spinning method described in FR 2 805 179 and WO 01/63028. That method consists of homogeneously dispersing the nanotubes in a liquid environment.
Once dispersed, the nanotubes are re-condensed in the form of a ribbon or a pre-fiber by injecting the dispersion into another liquid causing the nanotubes to coagulate.
The ribbons, pre-fibers or final fibers can be treated by stretching in a wet method to improve the direction of the nanotubes. These reshaping methods are described in FR 01 10611 as dynamically or statically stretching the fiber in solvents with a higher or lower affinity for the coagulating polymer, for improving the structure and the physical properties of the fibers.
The properties of these fibers, as those of any other fibers, depend in a critical manner on the nature and arrangement of their components.
Carbon nanotubes constitute an “intelligent” material capable of responding at a mechanical and electrical level to electrical, mechanical, chemical or biological stimuli. Nanotube fiber can constitute a structure particularly adapted for highlighting these functions. Indeed, the nanotube fiber constitutes a macroscopic object containing a large proportion of oriented nanotubes. Thus, it is possible to produce actuators with performance greatly surpassing that of other nanotube assemblies, such as nanotube paper. It is also possible to manufacture micro-electrodes which are much more sensitive than conventional micro-electrodes made from conventional carbon fibers.
Sensors or actuators based on “intelligent materials” are often used in devices to amplify their deformation. The most common example is the bimetallic strip made from piezoelectric material. These bimetallic strips consist of an active piezoelectric layer and a passive inert layer. When the piezoelectric strip stretches or contracts, the bimetallic strip curves sharply. A very slight deformation, on a microscopic scale, can thus be greatly amplified and viewed macroscopically. In the same way, there are sensors made out of very thin metallic sheets. The strip curves sharply in the presence of a particular chemical component adsorbing on one of its surfaces. This effect is the result of modifying different surface constraints on both surfaces of the thin sheet. As in the example of the piezoelectric bimetallic strip, the mechanical effect is greatly amplified by the asymmetry of the system. This significant amplification of the curvature comes from a slight stretching or contracting deformation.
Such bimetallic strips, functioning as sensors or actuators, can be made from nanotube films. However, the electromechanical and electrochemical properties of fibers being superior to those of films, the use of these fibers improves the intrinsic properties of the whole, which benefits from the orientation and density of the nanotubes in a fiber. Furthermore, this makes it possible to manufacture very small devices which can potentially be used as micro-sensors or micro-actuators.
The production of these bimetallic strips corresponds to an asymmetric structure of the nanotube fiber.
Some approaches have been tried to cover a carbon nanotube fiber in an asymmetric manner.
The fiber can be passed in front of a device which “paints” a single side, over the surface of a liquid in the form of a bath or a drop. This approach can, in certain cases, result in asymmetric coating, but does not allow the deposited amount to be easily controlled. The limitations of this approach relate to the wetting properties and the viscosity of the liquid used. If the liquid (solution, melted polymer) is not very viscous, it spreads quickly around the fiber and coating is not asymmetric. By using a more viscous system, it is possible to achieve asymmetric coating, but its thickness cannot easily be controlled. For example, for a polymer which remains very viscous in the melted state at high temperatures and solidifies at low temperatures, a certain amount of polymer is drawn and rapidly congealed during cooling. The drawn amount depends on several related parameters which are difficult to control: polymer viscosity, cooling, fiber passage speed, wettability of the fiber by the polymer. In the end, the coated amount is only slightly controlled. It might be considered that passing through a polymer solution, rather than through a melted product, could solve this problem. Indeed, by dissolving the polymer a system with controllable and reproducible viscosity and wetting properties is attained. These only depend on the solvent used and not on a cooling process. When the solvent evaporates, the polymer dries onto the. By using different concentrations of polymer, layers of controlled thickness can be made. However, passing through a solution does not work in a satisfactory manner as a low-viscosity solution has a tendency to spread spontaneously and uniformly around the fiber.